The present invention relates to a fusion splicing method for optical fibers.
The transmission loss of a quartz glass fiber has now been reduced as low as 0.2 dB/km (substantially equal to its theoretical value) at a 1.55 .mu.m wavelength; this has made possible transmission without any repeater over a distance of 100 km. Furthermore, it is predicted, theoretically, that the transmission loss of a fluoride glass fiber of ZrF.sub.4, HfF.sub.4 series will be 0.01 dB/km or less in a 2 to 4 .mu.m wavelength band, and it is now drawing attention as an optical fiber of the next generation.
With such a low transmission loss of the filter as mentioned above, an optical signal could be transmitted over several thousand kilometers without regenerative or amplifying repeating of the signal.
At present, however, the length of a one-piece fiber which can be produced even by the most advanced quartz fiber manufacturing technique is in the order of 100 km at best, and techniques capable of fabricating a fiber 1000 km or more in length have not yet been established. In addition, a fiber which has sufficient required strength in its entire length for incorporation into a cable is 10 to 20 km at the longest, so that in even a system of a 50 km or so transmission distance is forced to have at least several joints of optical fibers. Accordingly, as the transmission loss of the optical fiber is reduced, an optical fiber splicing technique for low loss and high strength splicing acquires a greater importance.
The most common method that is now employed for splicing optical fibers of quartz system is a fusion splicing method, in which two fibers to be spliced are butted tightly at one end and the butted end portions are fused together. With this method, the splice loss is low and very small in aging with time; hence this method has been one of factors in the development of optical communications so far. However, this method possesses a defect such that the strength of the joint decreases to 1/3 to 1/5 the strength of fiber strands. This is ascribed to the fact that water adsorbed on the fiber surface causes the formation of crystallites during the fusion splicing process.
Moreover, it is considered that fusion splicing of fluoride glass fibers is difficult because when heated in the air, fluoride glass crystallizes before it softens.
Factors contributing to the crystallization of the fluoride glass fall into the basic one in which the glass crystallizes by its own instability and the external one in which oxygen or water vapor contained in the atmosphere, or water adsorbed on the glass surface reacts with the glass to form crystal nuclei which grow into crystals.
With respect to the former, our studies on the viscosity temperature characteristics of glass of various compositions and the crystallization temperatures thereof have revealed that short-time heating in an inert gas atmosphere would not cause the crystallization as long as the composition of the glass used falls within the range in which fibers can be produced; this means that the fusion splicing of the fluoride glass fibers has no inherent disadvantage.
Therefore, it is necessary only to completely remove impurities such as water adsorbed on the glass surface, which forms the external cause of the crystallization, and to perform the fusion splicing in an inert gas atmosphere.
Also in the case of the quartz glass fiber, the removal of water adsorbed on the glass surface would suppress the formation of crystallites, permitting high-strength fusion splicing.
At present, heat treatment of the glass in a dry atmosphere or in a vacuum is the most effective for the removal of the impurities such as water adsorbed on the glass surface. However, this method cannot be applied to a material which reacts with water or like impurities and crystallizes even at low temperatures. Further, even the quartz glass fiber which is stable thermally is impossible of sufficient dehydration because heating of its butted portion to a temperature for dehydration will degrade its resin coating. Thus no effective means is available, at present, for completely removing the adsorbed water and like impurities from the glass fiber of its butted portion to obtain a clean glass surface.